
diff -r 06a04b3b2dda -r d7109c6e721d hws/hw05.tex
--- a/hws/hw05.tex	Thu Oct 30 01:17:51 2014 +0000
+++ b/hws/hw05.tex	Mon Nov 03 03:22:41 2014 +0000
@@ -1,26 +1,51 @@
 \documentclass{article}
-\usepackage{charter}
-\usepackage{hyperref}
-\usepackage{amssymb}
+\usepackage{../style}
 
 \begin{document}
 
 \section*{Homework 5}
 
 \begin{enumerate}
+\item What can attacker that controls the network do to a communication
+between a client and a server?
+
+\item Before starting a TCP connection, client and servers
+perform a three-way handshake. Describe how can this three-way
+handshake can be abused by an attacker? 
+
 \item Consider the following simple mutual authentication protocol:
 
 \begin{center}
 \begin{tabular}{ll}
-$A \rightarrow B$: & $N_a$\\  
-$B \rightarrow A$: & $\{N_a, N_b\}_{K_{ab}}$\\
-$A \rightarrow B$: & $N_b$\\
+$A \to B$: & $N_a$\\  
+$B \to A$: & $\{N_a, N_b\}_{K_{ab}}$\\
+$A \to B$: & $N_b$\\
 \end{tabular}
 \end{center}
 
 Explain how an attacker $B'$ can launch an impersonation attack by 
 intercepting all messages for $B$ and make $A$ decrypt her own challenges.
 
+\item What is the main problem with the following
+authentication protocol where $A$ sends $B$ mutually
+shared key?
+
+\begin{center}
+$A \to B: K_{AB}$
+\end{center}
+
+\item Nonces are unpredicatble random numbers used in protocols? 
+Consider the following protocol
+
+\begin{center}
+\begin{tabular}{ll}
+$A \to B$: & $N$\\  
+$B \to A$: & $\{N + 1\}_{K_{ab}}$\\
+\end{tabular}
+\end{center}
+
+Write down three facts that $A$ can infer after this protocol has been
+successfully completed?
 
 \item Before starting a TCP connection, client and servers
 perform a three-way handshake:
@@ -35,6 +60,29 @@
 
 How can this protocol be abused causing trouble on the server?
 
+\item Write down a protocol which establishes a secret key
+between $A$ and $B$ using a mutually trusted third party $S$.
+You can assume $A$ and $S$, respectfully $B$ and $S$, share 
+secret keys.
+
+\item Consider the following protocol between a car and a
+key transponder: 
+
+\begin{enumerate}
+\item $C$ generates a random number $r$
+\item $C$ calculates $(F,G) = \{r\}_K$
+\item $C \to T$: $r, F$
+\item $T$ calculates $(F',G') = \{r\}_K$
+\item $T$ checks that $F = F'$
+\item $T \to C$: $r, G'$
+\item $C$ checks that $G = G'$
+\end{enumerate}
+
+In Step 2 and 4 a message is split into two halves. Explain
+what the purpose of this split is? 
+
+
+
 \end{enumerate}
 \end{document}
 




